Java metamodel

The Java metamodel : it is the reflection of the Java language, as defined in version 3 of "Java Language Specification" from Sun Microsystems ("JLS3" corresponds to JDK 5).

The Java metamodel contains 126 metaclasses.
To better understand it, this page will introduce its main features (metaclasses and links).

You could also browse the model definition, java.ecore available in sources (see install section).

A good tip is also to see the javadoc associated to the metamodel implemented by the Eclipse team in the JDT (Java Development Tool) project. This metamodel and Java are very similar.

Main metaclasses

ASTNode

Every metaclass (apart from the Model metaclass) inherits from ASTNode.
As its name indicates, ASTNode represents a graph node.
ASTNode has a reference to the Comment metaclass because almost every java element can be associated to a comment (block or line comment and Javadoc). More details in the "Comment" section.

ASTNode metaclass

Model, Package, AbstractTypeDeclaration

The root element of each Java model is an instance of the "Model" metaclass. It is a translation of the concept of java application, so it contains package declarations (instances of the Package metaclass). And package declarations contain type declarations (instances compatible with the AbstractTypeDeclaration metaclass), and so on...

Model, Package & type declaration superclass

NamedElement & notion of Proxy

A lot of java elements are "named", and this name could be considered as an identifier : methods, packages, types, variables, fields, ...
So all the corresponding metaclasses inherit from the NamedElement metaclass.

Another goal of this metaclass is to indicate whether an element is part of the current Java application or not (element from an external library of from the JDK). So, external elements are tagged as proxy through a dedicated attribute and can be easily filtered.
For example, instruction "System.out.println" has been decomposed into three named elements (one class, one variable and one method) the definitions of which are not part of the current Java application. So attribute "proxy" of these elements has been initialized to true.

NamedElement and its hierarchy

TypeAccess, PackageAccess, SingleVariableAccess, UnresolvedItemAccess

To represent links between Java elements, metaclasses TypeAccess, PackageAccess, SingleVariableAccess and UnresolvedItemAccess were introduced. Each allows to navigate directly to a NamedElement (proxy or not) in the graph.

A TypeAccess represents a reference on a type.

TypeAccess

A PackageAccess represents a reference on a package.

PackageAccess

A SingleVariableAccess represents a reference on a variable.

SingleVariableAccess

On the contrary, references to methods are direct.

MethodInvocation

BodyDeclaration

A type declaration has different kinds of contents : fields, methods, static block, initialization block or other type declarations. All of these elements are of type BodyDeclaration metaclass.

BodyDeclaration and its hierarchy

Expressions

Like in many languages, the concept of expression exists in Java : it is a portion of code, without declarations, and its evaluation returns a value, numerical or boolean or other ...

For example,

++i

is an expression and it will be translated to the concept of PrefixExpression metaclass.

All types of expressions shall inherit from the Expression metaclass.

Expression and its hierarchy

Statements

An "instruction" in Java is represented by the Statement metaclass. A block of code (Block metaclass) contains a collection of statements, and a block of code may be contained by a method.

Some examples of statements in java :

if, while, for, do, ...

All of their definitions use the concept of expression to separate the value from the instruction keyword.

Statement and its hierarchy

Zoom on comments

There are three kinds of comments : line comments (//), block comments (/* */) and javadoc comments (/** */).
Every comment contains text fragments, but we can have more information on javadoc comments (Javadoc metaclass): they can also contain javadoc tags that are identified and collected through instances of the TagElement metaclass.

Comment and its hierarchy

Zoom on annotations

Official usage of annotations has been introduced in version 5 of the JDK.
To handle them, annotation declarations are managed as type declarations (AnnotationTypeDeclaration metaclass) with specific attributes (AnnotationTypeMemberDeclaration metaclass).
And annotation usages are translated into instances of the Annotation metaclass, which reference corresponding annotation declarations. And parameters are translated into instances of the MemberValuePair metaclass.
Some statements also have a list of annotations.

Annotation

Zoom on generics

Version 5 of the JDK also introduced the concept of "generics".
Generic types of method declarations are translated into instances of TypeDeclaration of AbstractMethodDeclaration metaclass with arguments as instances of the TypeParameter metaclass.
Uses of these generics are translated into instances of the ParameterizedType metaclass which reference concrete elements (type and type arguments).

A specific case of type argument is the "wildcard" (for example "<? extends X>"). There is a specific metaclass to handle it : WildCardType.

Generics

Requirements

To use the plug-in you need:

JDK 1.5 or above

a version of Eclipse 3.3 or above with the following set of plug-ins installed

Associated Discoverers

Java Discoverer

The goal of the Java Discoverer plug-in is to allow practical extractions of Java models from Java projects.

Description

This plug-in aims at analyzing Java source code compliant with version 3 of "Java Language Specification" from Sun Microsystems and providing a model describing the information found, conforming to the Java metamodel.

A Java model contains the full abstract syntax tree of the Java program: each statement such as attribute definition, method invocation or loop is described. In addition, links between elements are resolved (by resolved link we mean for instance a link between a method invocation and the declaration of this method, or between the usage of a variable and its declaration). The model can thus be seen as an abstract syntax graph (ASG).

Requirements

To use the plug-in you need:

JDK 1.5 or above

a version of Eclipse 3.3 or above with the following set of plug-ins installed

User manual

The plug-in provides the user with a contextual menu to easily create models.

By right-clicking on a Java Project in the Eclipse Package Explorer view, you can quickly create the Java model of your application (see next Figure).

Menus in Eclipse to create model from source code

A progress bar will appear at the bottom of the window as soon as the operation begins. Depending on the size of your application, the reverse engineering process might take some time to complete (see next Figure).

Progress bar during model creation

At the end of the process, the newly created model files are added to the root of your project and are automatically opened in the default editor (see next Figure).

Java model in the package explorer

.javaxmi files can be opened in the Sample Ecore Model Editor (see next Figure).

Java model viewed with EMF browser

They can also be opened in the MoDisco model browser (see next Figure) which brings some graphical improvements.